Diving tank – structure and properties. Everything you need to know.

A diving tank is a must-have item for diving equipment. For the basic diving course you will use compressed air that will be in the tank. But what exactly is a diving tank and how is it built?

The diving tank is a tank adapted to store compressed gas. Diving cylinders are usually made of steel or aluminum. In recreational and technical diving we usually use cylinders made of these two materials. The steel diving cylinder, compared to the aluminum cylinder, has a smaller size while maintaining the same water capacity.

Steel cylinders are harder and have much greater resistance to any mechanical damage. Wait a minute … why am I writing about water capacity? We store air in bottles, not water. We will explain everything to you immediately.

Diving tank – water capacity of gas cylinders

This term means the amount of water that would fit inside an empty cylinder when you unscrew the valve. Usually steel cylinders with a capacity of 10 liters, 12 liters and 15 liters are used for recreational divers. Sometimes you will probably also find 18 liter cylinders. However, due to their diameter and weight, they are rarely used.

In the United States, cylinder capacities are given in cubic feet. This is the amount of air that can be compressed in a specific dive cylinder to 3000 PSI (pound per square inch). Therefore, typical capacities of aluminum cylinders are: 40, 80, 100, 120.

Steel or aluminum diving tanks?

Steel dive cylinders are more susceptible to corrosion. Especially if we use them in sea water. A steel cylinder can rust not only from the outside but also from the inside. This will happen when water gets inside. And it doesn’t matter if it’s sweet or salty. The steel diving tank corrodes very quickly, which is why it is very important and important to react quickly to any minor damage.

Aluminum diving tank.

The aluminum cylinder is lighter than the steel cylinder. This is useful when you need to carry a large amount of cylinders or when every kilo of luggage counts. Unfortunately, aluminum cylinders also have their drawbacks. Aluminum cylinders are more prone to damage compared to steel cylinders.

Their walls are thicker, making the aluminum cylinder larger than a steel cylinder of the same capacity. Aluminum diving cylinders are also subject to corrosion, but this is different from steel cylinders. Aluminum does not rust, but it is covered by oxidation.

How is the diving tank built?

Whether you’re dealing with a steel or aluminum cylinder, you’ll find all the information on its cup. But where is the cup? For this we can perfectly use the illustrative picture below. Each cylinder consists (from the top of the cylinder) of a neck, cup, mantle (working part) and bottom.

Diving-tank structure-and-properties.-Everything-you-need-to-know.-4

  1. Cylinder neck
  2. Cylinder cap
  3. Cylinder coating
  4. Cylinder bottom

The neck of the tank is located on top of the diving tank. It is inside the thread that we then screw in the right type of valve.

The cup is a place where all information related to a given dive cylinder is placed. It is made of a thicker part of the material and it is here that we will find permanent cylinder markings. What can we find here? Among other things, information about the type of thread in the neck of the cylinder, the material from which it is made, the serial number, working pressure and test pressure or the date of the hydrostatic test.

Each diving tank also has its bottom. It is made of the thickest layer of material, because it is this part that is most vulnerable to scratches and damage caused by its placing and moving. The steel cylinder usually has a round bottom, which prevents it from standing freely. That is why a special “shoe” is very often used, which protects the bottom of the cylinder against damage.

Markings on the cylinder cup

At first glance, it may seem that the information on the cylinder cup is a collection of random letters and numbers. However, this is very important and structured information. There you will find information such as:

  • Working pressure
  • Hydrostatic pressure test
  • Water capacity
  • Cylinder weight
  • Production date
  • Date of previous technical inspection

How to take care of the diving tank?

Many people wonder how to care for cylinders, how to use them correctly, how to make their use as long as possible and as safe as possible. In the end they work this enormous pressure. Below are some insights on caring for diving tanks.

In recreational or technical diving we practically deal with two materials from which the cylinder shell is made. Steel and aluminum. Steel cylinders are more exposed to corrosion, especially when dealing with salt water. To prevent this, manufacturers protect the surface in a variety of ways – electroplating, anti-corrosive paint, etc. With aluminum, both painted and unpainted are found due to smaller and other corrosive properties.

If the cylinder was filled with good quality air and was not very battered, its durability often exceeds 10 years without major repairs. However, if the paint coating suffered, the cylinder was not cleaned, and the air from the compressors was of poor quality, then such a cylinder should be repaired. Rust from the cylinder contaminates the regulator and, consequently, damages it.

An important element of the cylinder is the valve. Its proper operation allows for safe and pleasant diving, because we are sure of its tightness, easy opening and closing. We need to be sure that the cylinder valve has been fully opened.

Without a complete opening of the valve, as the depth increases, and hence the air flow, we can have a problem with taking a breath from the regulator. Which becomes especially dangerous at greater depths and in stress, where the air flow is even greater.

Diving tank - structure and properties. Everything you need to know.
Diving tank – structure and properties. Everything you need to know.

A special case that may expose the diver to danger is the neglect of the diverter valve. Its performance is rarely checked, it is almost always in the open position. It would not be good if it failed when it was used for the purpose for which it was created – cylinder separation in e.g. failure.

The signal that must alarm us in the operation of the valve is “skipping” the knob, resistance to its unscrewing and turning, and a large force necessary to tightly turn the cylinder. Therefore, the cylinder valve should also be checked prophylactically at the diving service.

Diving tank valve

It is very important for the safety of use to know the difference in cylinder valve threads. There are two very similar visual standards. Metric thread m25x2 and inch thread BSP 3/4, so slightly differing in diameter and thread pitch that there is a possibility of confusion.

Diving cylinders should be stored in any position with minimal excess pressure. There should be several bars of air in the cylinder. It is absolutely unacceptable to knock the cylinder, causing deformation on the surface. Also, very severe corrosion pits must not be allowed to form. Such a cylinder will only be scrapped.

Caring for cylinders is extremely important. The diving tank with the breathing automaton is the most important elements of the diving set ensuring the diver’s safety. We must take care of the cylinder valve, its tightness and proper functioning. Rust on its outer shell and inside, which in consequence affects the destruction of the regulators is also not accepted.

Filling the diving tank

Filling the diving cylinder is equally important. Only compressors for breathing air should be used for this purpose. You cannot fill the cylinder with automotive or industrial compressors. Such air may be contaminated and threaten your safety. The person filling diving tanks must have training and experience.

Diving tank – structure and properties. Summary

Diving with equipment requires the use of a diving cylinder. This is a very important element that should be properly cared for. The diving tank can be used for many years and be with you during many dives.


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